How to limit PDF rasterization size when using ImageMagick/Ghostscript - pdf

Question

To convert pdfs/eps to pixel images we use ImageMagick. When converting a PDF the command can look like this:
convert -verbose -density 150 -trim -colorspace sRGB input.pdf -quality 90 -flatten -sharpen 0x1.0 output.png
However, the input PDF (which contains only a few paths) has a specified size of 300cm by 200cm, and Ghostscript doesn't like this, and creates a huge png. The verbose output of imagemagick shows this:
"gs" -q -dQUIET -dSAFER -dBATCH -dNOPAUSE -dNOPROMPT -dMaxBitmap=500000000 -dAlignToPixels=0 -dGridFitTT=2 "-sDEVICE=pngalpha" -dTextAlphaBits=4 -dGraphicsAlphaBits=4 "-r150x150" "-sOutputFile=/tmp/magick-3036AW7mUOP25w7J%d" "-f/tmp/magick-3036PxgJinljqMwV" "-f/tmp/magick-30369hcErAROr7V6"
/tmp/magick-3036AW7mUOP25w7J1 PNG 17717x11811 17717x11811+0+0 8-bit sRGB 1.003MB 3.910u 0:03.929
input.pdf PNG 17717x11811 17717x11811+0+0 16-bit sRGB 1.003MB 0.000u 0:00.000
PNG 17717x11811. Huge. I only need a 256x256 image.
I have tried the geometry (-geometry 256x256) options and the density option (before the filename, -density 150) in different configurations, but I does not change the ghostscript output, for example:
convert -verbose -density 150 -trim -geometry 265x265 -colorspace sRGB input.pdf -quality 90 output.png
"gs" -q -dQUIET -dSAFER -dBATCH -dNOPAUSE -dNOPROMPT -dMaxBitmap=500000000 -dAlignToPixels=0 -dGridFitTT=2 "-sDEVICE=pngalpha" -dTextAlphaBits=4 -dGraphicsAlphaBits=4 "-r150x150" "-sOutputFile=/tmp/magick-3233p3ofct0fiy5T%d" "-f/tmp/magick-3233Rads_vSSpKa6" "-f/tmp/magick-3233LQMrrEFgT0fi"
/tmp/magick-3233p3ofct0fiy5T1 PNG 17717x11811 17717x11811+0+0 8-bit sRGB 1.003MB 3.800u 0:03.799
input.pdf PNG 17717x11811 17717x11811+0+0 16-bit sRGB 1.003MB 0.000u 0:00.000
input.pdf=>output.png PNG 17717x11811=>265x38 321x213+28+65 16-bit sRGB 7.96KB 1.080u 0:00.559
How can you define the constraints for Ghostscript, when using ImageMagick?

Answer 1

I wouldn't say 'Ghostscript doesn't like this', if the media size is huge, then of course Ghostscript creates a huge PNG, what else did you expect it to do ?
300 cm is 118.11 inches, which at 150 dpi works out at a bitmap 17716.5, or rounded up to 17717 pixels.
200 cm is 78.74 inches, at 150 dpi that works out to 11811 pixels.
If you want it at a lower resolution, then alter the setting of -r ('density' in ImageMagick). For example, you could set 'density 10', presumably that would produce a file 1181x787. To get 256x256 you would need to set the resolution to ~2 dpi. You may, of course, find that its rather hard to see any detail when the result is that coarse.
Alternatively you can tell Ghostscript the size of media you want, and tell it 'FIXEDMEDIA' so it doesn't alter according to requests from the PostScript program or PDF file.
-g sets the media dimensions in pixels, and -dFIXEDMEDIA tells Ghostscript that the media is fixed. You will almost certainly want to also set -dFitPage, or you will only get a tiny portion of the bottom left hand corner. You will also need to not set -r.
-dFIXEDMEDIA -g and -r are described in use.htm in the ghostscript documentation
Depending on the age of your Ghostscript installation you may not be able to use -dFitPage but you might have to use -dPDFFitPage.
Almost certainly you will want to do this from the command line using Ghostscript instead of ImageMagick, I imagine constructing the command line in IM would be difficult.